I think mine must have been when I first got access to WFC on my DS.
I went to the GTS and for weeks, all I did was trade on the GTS for random stuff.

I got my first event Pokémon, MT BATA Ho-oh (French Colosseum Ho-oh.) Then later, got several Aura Mew, then several Wishmaker/Channel Jirachi (I think I got a Negai Boshi Jirachi once and put it back on the GTS, thinking it was a hacked Jirachi. )